The present invention relates to an apparatus for locking relative sliding movement of first and second parts. In particular the present invention relates to a measuring instrument for use in surgery including such an apparatus.
In orthopaedic surgery, for example knee replacement surgery, parts of a joint which are damaged are replaced with a prosthesis. In knee replacement surgery one such prosthesis is a femoral prosthesis which is affixed to the femur to provide a replacement bearing surface for the knee joint. Before fitting the prosthesis it is necessary to prepare the femur for the implant by cutting a receiving surface on the bone and locating the correct position for positioning pins. The femoral prosthesis is available in a number of different sizes to suit the anatomy of the patient. A measuring instrument is used to determine the size of the implant and to ensure that any holes for receiving positioning pins of a prosthesis are in the correct position.
A typical known femoral sizing guide is an instrument made entirely of metal or metal alloy. It comprises parts that can slide relative to one another to measure the anatomy of the patient and determine the correct size of implant. Once the correct size has been determined, it is desirable to lock the parts, so that they cannot move relative to each other. This is achieved using a knob-based friction lock. A threaded bore is provided in one part above a surface on the other part. A threaded knob is then screwed into the bore so that one end protrudes out of the bore and further rotation of the threaded knob into the bore causes it contact and press on the other part, locking the two parts against relative movement by friction. As the instrument is made of metal there is no significant compression or deformation when the threaded knob is used to lock the parts against relative movement
This friction based locking using threaded knobs can be cumbersome to use with gloved hands in a fluid filled wound in the operating room. However, with alternative constructions it can difficult to provide sufficient friction to lock the two parts against relative movement without the mechanical advantage of the threaded knob.